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-   -   Re: Retrieving an object from a set (http://www.velocityreviews.com/forums/t956944-re-retrieving-an-object-from-a-set.html)

Ethan Furman 01-25-2013 11:38 PM

Re: Retrieving an object from a set
 
On 01/25/2013 03:14 PM, Arnaud Delobelle wrote:
> I've got a seemingly simple problem, but for which I cannot find a
> simple solution.
>
> I have a set of objects (say S) containing an object which is equal to
> a given object (say x). So
>
> x in S
>
> is true. So there is an object y in S which is equal to x. My
> problem is how to retrieve y, without going through the whole set.
> Here is a simple illustration with tuples (my actual scenario is not
> with tuples but with a custom class):
>
>>>> y = (1, 2, 3) # This is the 'hidden object'
>>>> S = set([y] + range(10000))
>>>> x = (1, 2, 3)
>>>> x in S

> True
>>>> x is y

> False
>
> I haven't found y. It's a very simple problem, and this is the
> simplest solution I can think of:
>
> class FindEqual(object):
> def __init__(self, obj):
> self.obj = obj
> def __hash__(self):
> return hash(self.obj)
> def __eq__(self, other):
> equal = self.obj == other
> if equal:
> self.lastequal = other
> return equal
>
>>>> yfinder = FindEqual(x)
>>>> yfinder in S

> True
>>>> yfinder.lastequal is y

> True
>
> I've found y! I'm not happy with this as it really is a trick. Is
> there a cleaner solution?


I don't know if there is a cleaner solution, and I quite like yours.

Can you tell us, though, why you have to have y if x == y? Is there
some subtle difference between the two equal objects?

~Ethan~



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